X-linked hypophosphatemic rickets is characterized from a dental point of view by dentin hypomineralization, associated with enlarged pulp chambers, prominent pulp horns, and spontaneous pulp necrosis with periapical infections. Objectives: Our aim was to further characterize the dentin structure associated with correct and defective 1alfa-(0H) vitamin D treatment. Methods: Three groups were considered: hypophosphatemic children with good compliance to therapy (n= 3), hypophosphatemic children with poor compliance to therapy (n= 3) and control age-matched patients (n=6). Clinical and radiographic parameters were recorded. Extracted teeth were fixed, cut longitudinally with a diamond saw (Acutom, DK) and prepared for SEM examination. Results: Clinical parameters have been previously reported (Chaussain Miller et al, 2003). Direct SEM and backscattering examination revealed that in pathologic teeth, the dentin outer layers were identical to the controls: the coronal mantle dentin, the radicular Hopewell Smith and Tome’s granular layers. The circumpulpal dentin was severely disturbed in poorly compliant hypophosphatemic patients in deciduous teeth as well as in permanent teeth. Large interglobular defects appeared between calcospherites. For compliant patients, the outer circumpulpal dentin was altered, but to a lesser degree than in non-compliant patients, whereas the inner layer was normal. Conclusions: We conclude that, firstly, the formation of the peripheral dentin layers are not under the control of the genetic disease, hence unrelated to the phosphate status of the patient. Secondly, the target is the circumpulpal dentin. Thirdly, even if the 1alfa-(0H) treatment has a beneficial impact on the dental structure of hypophosphatemic patients, structural alterations are still observed, revealing that dentin is a good indicator for this genetic disorder.